1. Field of the Invention
The present invention relates to a green optical filter phosphate-based glass having a strong near infrared absorption and a high transmittance around the peak of about 555 nm and with the steepest slope of the absorption curve occurring between about 570 nm to 650 nm.
2. Description of the Prior Art
In order to obtain a green optical display filter that is compatible with night vision imaging systems, the filter must have a strong near infrared absorption to prevent the display lighting from interfering with the operation of night vision goggles which is quite sensitive to near infrared radiation. This type of display filter is required to have a transmittance of less than 10.sup.-6 for near infrared radiation of 700 nm to 950 nm and less than 5.times.10.sup.-6 at 650 nm to 700 nm. At the same time, it should have a transmission peak for green radiation of 550 nm to 565 nm.
One conventional approach to achieving these requirements involves costly and laborious arrangements of cementing a polished infrared suppressive glass filter onto another polished color correction glass filter. However these two glasses must be selected so as to have similar if not identical thermal expansion properties in order for the cement to continue to hold the glasses together through temperature and weather variations. The additional color correction layer will also cause further inconvenience when the display demands a thin window filter.
The use of alkali metal or alkaline earth metal phosphate glass used as host glass for colored optical filters has also been proposed wherein the phosphate glasses contain CuO as a coloring agent, e.g. see U.S. Pat. Nos. 4,303,298, 4,110,245 and 4,615,989 in this regard.